Short Circuit Rating

as per IEC-60909-0 there is I"k I.e. Initial symmetrical short circuit current (rms) and it does not define I'k. Please clarify/recheck. However, IEC defines ip i.e. Peak short circuit current. Switchgear components shall be selected as per initial symmetrical short circuit current as well peak SC current. Eg. If your short circuit level is 50 kA then your peak SC become 50*2.1=105kA. I.e. ACB/MCCB shall be suiutable for rated fault level of 50 kA and also for peak SC level of 105 kA. However, bus bars / bus ducts shall be designed as per rated fault level I.e. In our case 50 kA. Switch gears are defined in terms of rated service SC breaking capacity Ics, rated ultimate SC breaking capacity Icu, and rated short time withstand current Icw. A best selection of switchgear is Ics=Icu=50kA in our case. Icw is applicable for switch gears of utilisation category B I.e. Those having settable time delay (eg. ACB). Icw shall also be 50kA for 1sec of switchgear selected, in our case. There is also rated short time making capacity Iqm (peak value) = k x sqrt(2) x I"k in this equation k is R/X ratio of system k=1.02+0.98e^(-3R/X). In summary for MV system Peak SC factors to I"k are 2.54 for MV systems, for systems with fault current unto 50kA it is 2.1 and for above it is 2.2 times. Now you can do selection of your switchgear once you calculate fault current. As u know selection of MCC is having to aspects 1 selection of switchgear and 2 selection of switchgear. Hope I have answered your both questions...!

What method did you use the calculate the fault current?

@Salunkhem: Actually it's Ik, Steady-State Short Circuit Current. That means I have to use the 100kA panel then. I have increase the transformer impedance from 6% yo 9%, but the short circuit level is still more than 80kA. Is there a cheapest method to bring down the I"k slightly below 80kA?

@wareagle: The method is IEC 60909, exclude the Duty Calc.

Can you send your single line diagram on personal mail provided by cr4 I will have a look at it and crosscheck the results and let you know the solution. Prima-facie there seems to be something not appropriate. And yes, IEC itself said that, there are only two methods per unit and actual impedance calculation, whichever you do you should get same answer.

Move your MCC farther from your transformer. The longer cable length will provide more impedance, and the fault level at the MCC will be lower. You would have to rework your analysis to find out how far, but it's been done before.

I know of one situation where the engineer just looped cable around the transformer vault until he had enough impedance to drop the fault current to the value he wanted. That may be the cheapest way for you to do it. Similar to PeterT's suggestion.

I've read about using a Fault Current Limiter between the Transformer and MCC. The question is what would be the final kA rating? Because the specification only mention about the kA Rating (120 kA which is way bigger than the MCC kA rating of 80kA) and the load ampere.

Is there somebody have an experience with Fault Current Limiter or Reactor to limit the fault current?

Regards

Yep in one of power plant supplied...

As what I read Ikmax (maximum steady state shortcicuit current) is used to determine minimum device rating. I don't see the sense! because I will rather choose of course I"kmax to determine max. device rating and this is the big deal to me not the min. device rating.

Ikmin (minimum steady state short circuit) used in setting of relay or protection

Ik"max, Ikmax, I"kmin and Ikmin is define by cmax & cmin factor in IEC 609090-0

One thing I"k is not used in comparing Device Duty, I"k is used to determine Ib and Ip and these are what we need.